1. Field of the Invention
The present invention describes a process for the removal of cadmium ions from phosphoric acid resulting from a wet process, in the following designated as "wet process phosphoric acid", preferably wet process phosphoric acid, obtained by dissolution of crude phosphate ores with sulfuric acid, said process being carried out by a liquid-liquid extraction using salts of alkyl amines or ammonium bases with anionic chlorocomplexes of polyvalent metal ions, which are dissolved in an inert organic solvent.
2. Brief Description of the Background of the Invention Including Prior Art
Cadmium belongs to the heavy metals whose spread is to be avoided. Crude phosphate ores used in the preparations of phosphate-containing fertilizers contain cadmium. By dressing the crude phosphate with sulfuric acid, said cadmium in the form of an easily soluble salt enters the produced phosphoric acid, and when the latter is further processed into fertilizers, it also enters into the same.
There are differences in cadmium contents of the various crude phosphate ores. The attempts have been numerous to reduce the cadmium content of crude phosphoric acids which have been prepared from crude phosphate ores having a high cadmium content.
However, the removal of cadmium from a crude phosphoric acid involves considerable difficulties. Cadmium salts are characterized by a very good solubility, in particular in acids of higher concentration.
Usually, cadmium is precipitated from weakly acidic aqueous solutions as a difficultly soluble sulfide by treatment with hydrogen sulfide.
According to German Patent Application Laid Open DE-OS 2,422,902, it is possible to precipitate cadmium from phosphoric acids of higher concentration and having a P.sub.2 O.sub.5 content of more than 45 weight-percent under an excess pressure of up to 50 at and with a residence time of up to 3 hours. The examples of said German Application Laid Open show that, after completion of said treatment, about 10 to 20 percent of the originally present cadmium still remain in the acid. Therefore said process leads to a reduction of the cadmium content but not to an essentially complete removal of cadmium from the phosphoric acid.
Furthermore, it is possible to remove cadmium from crude phosphoric acids by extraction with organic solvents. The multitude of known purification processes of crude phosphoric acids show technical routes for this. In said process, a pure phosphoric acid is obtained in which all other cations, besides cadmium, are also largely removed from the phosphoric acid. In their purity, the acids are comparable to thermal phosphoric acid. By the high quality requirements, said processes, however, are cumbersome and extremely cost-intensive. For reasons of price, the thus obtained pure acids may, in general, not be used for the preparation of fertilizers.
British Patent GB-PS 1,053,929 describes a process for the removal of iron from phosphoric acid with the aid of primary, secondary, or tertiary amines which are dissolved in a hydrocarbon solvent. Phosphoric acid solutions, obtained by dressing phosphate ores with HCl, are subjected to said process, the iron being removed in the form of an anionic chlorocomplex.
The German Patent Application Laid Open DE-OS 3,218,599 describes an extraction procedure for the removal of cadmium from acidic, especially P.sub.2 O.sub.5 -containing solutions. As extractant, amine salts of hydrogen halide acids, in particular of hydrochloric acid, which are dissolved in an organic solvent, are used. As re-extractant, water or aqueous solutions of salts of oxyacids, in particular of ammonium salts of oxyacids, in the pH range between weakly acidic and neutral are used.
Said process is operated as multistage countercurrent extraction procedure. For the extraction stage, usually four countercurrent extraction units, and for the re-extraction stage, three countercurrent extraction units are necessary. For the preparation of the reagents, two further extraction units are required. The great number of necessary extraction units therefore leads to a high technical expenditure together with high investment costs.